Moon shadows and lunar far side samples headline the year
By James D. Thorne|December 2024
The Astrodynamics Technical Committee advances the science of trajectory determination, prediction and adjustment as well as spacecraft navigation and attitude determination.
In April, millions of people around the world traveled to North America to witness the solar eclipse, visible on April 8 in its totality across a slice of eastern Canada, the United States and central Mexico. In orbit, NASA said that the astronauts on the International Space Station had several opportunities to observe the eclipse. Scientifically, a total eclipse of the sun allows for studies of the sun’s corona and its gravitational lensing effect on the starlight coming from behind it.
A natural alignment of three celestial bodies such as the sun, moon and Earth, known as a syzygy, can take decades to repeat since they do not lie in the same plane except on rare occasions, so scientists have devised multiple ways to block the sun’s rays and form an artificial eclipse. For example, NASA’s New Horizons space probe in 2015 used Pluto to block the sun so that its instruments could detect the molecules that comprised the dwarf planet’s tenuous atmosphere. In February, the European Space Agency completed final integration of its Proba-3 satellites, which will use yet another technique to observe the sun’s corona: occulters, circular lenses installed on space-based telescopes to block the sun’s photons from reaching their detectors. However, placing an occulter too close to a telescope increases the potential for light diffraction, in which sunlight spills over the edge of the lens and blocks out fainter signals. So ESA plans to fly the two Proba-3 satellites in formation — one spacecraft with the occulter, the other with a coronagraph telescope. After their launch aboard an Indian rocket, scheduled for December, the satellites are to fly 144 meters apart in similar elliptic orbits, thus creating an eclipse for roughly six hours near each apogee, when the occulter spacecraft casts a shadow on the spacecraft with the coronagraph. Because of these slightly different orbits, the spacecraft will require small maneuvers to maintain the proper alignment to study the sun as it forms repeatable eclipses.
In June, China retrieved the first samples from the far side of the moon and returned them to Earth. This Chang’e-6 mission began in March with the launch of the Queqiao-2 relay satellite. The moon always keeps one face away from Earth due to tidal locking, so some form of satellite is required to relay communications to any lander touching down on the far side. Queqiao was placed in a highly elliptical frozen orbit, meaning one that naturally maintains its orientation due to the moon’s uneven gravity field combined with perturbations from the Earth’s gravity. Then in May, the Chang’e-6 spacecraft comprised of a lander and orbiter, among other components, was launched toward the moon. The lander and its ascent stage in June touched down in the South Pole-Aitken Basin, an ancient lava field on the lunar far side, where the lander drilled into the surface and picked up regolith with its robotic arm. Nearly 2,000 grams were collected and placed into a return capsule, which touched down under parachutes in Mongolia in late June.
Chang’e-6 marks the second time China has landed a robotic craft on the far side of the moon; the first lander, Chang’e-4, touched down in 2019. The 12 Apollo astronauts who visited the lunar surface landed generally in the equatorial region of the lunar near side, although there were proposals to visit the far side during the planned Apollo 20 mission before the Nixon administration canceled that landing and two others in the early 1970s.
